Om tat sat – According to Bhagavad Gita

Chapter 17, Text 23:

Three vedas here should mean three divisions: karma, jñāna, and upāsana (bhakti). This mantra “Oṁ tat sat” is advised to be used in all three contexts.

Another meaning in these words is:

The three smṛti state that “Oṁ tat sat” indicates the Supreme.

This is because “Oṁ” is a name for the Supreme Being, because “Oṁ” is the seed of all vowels, and thus the seed of all sound – just as the Supreme is the seed of all existence.

brāhmaṇās tena vedāś ca yajñāś ca vihitāḥ purā

Brāhmaṇas have always used this mantra to comprehend the Vedas and perform Yajña.

This shows that the mantra is to be used for jñāna-yoga (“to comprehend the Vedas”) and for karma-yoga (“to perform Yajña”). It is equally applicable to both because it is upāsana / bhakti. It is an aṅga of bhaktibecause it is brahman nirdeśa, a name for the Supreme.

Text 25

The word “tat” indicates that all ceremonies, charities, austerities, and deeds should be done without attachment to conventional results, but with the desire for mokṣa [enlightenment].

“Tat” is simply a pronoun, meaning “that.” But it has this meaning because it follows “Oṁ.” The “that” pronoun points to things. When it follows “Oṁ” it points us to god, to spiritual union with god, mokṣa.

Text 26

sad-bhāve sādhu-bhāve ca sad ity etat prayujyate

“Sat” is used in the sense of something that is real, and something that is good.

praśaste karmaṇi tathā sac-chabdaḥ pārtha yujyate

[So] use the word “Sat” to incline yourself to good deeds.

Text 27

yajñe tapasi dāne ca sthitiḥ sad iti cocyate

Become fixed in ceremony, austerity, and charity by pronouncing “Sat.”

karma caiva tad-arthīyaṁ sad ity evābhidhīyate

And, “Sat” indicates that your deeds should be dedicated only to real results.